1. Field of the Invention
The invention relates generally to selective pulse cancellation in a train of signals, and more particularly, to such systems as MTI radar in which a moving target is identified among plural fixed targets by accurate comparison of signals between successive received signal trains.
2. Description of the Prior Art
The instrumentations and techniques for MTI radar had their beginning relatively early in the radar arts. In general, the purpose of moving target indication radar (MTI) is the rejection of signals from fixed unwanted targets such as terrain features and buildings or other manmade structures, and the retention for detection or display, only those signals from moving targets such as aircraft. The elimination or substantial elimination of fixed target responses from a radar display is a great aid in the observation and tracking of moving targets.
A relatively up-to-date summary of the published state of the art in MTI radar is contained in Chapter 17 of the text "Radar Handbook" by Merrill I. Skolnik (McGraw Hill 1970). The material therein is supported by generous bibliographical references, so that the reader not entirely familiar with the relatively current state of the prior art can inform himself accordingly.
Although most MTI radar devices involve electronic processing either in the analog or digital category, it is also possible to effect optical processing as will be seen hereinafter in the description of the present invention.
Prior art analog processing for fixed target cancellation in MTI radar systems has been limited by the very stringent stability requirements, particularly for the delay device involved. Use has been made of the manysided quartz crystal operating as a resonant device broadbanded by mechanical means. Such devices operate at IF frequencies, and involve cancellation on an IF cycle-to-cycle basis. Any change in delay (phase) characteristics causes a deterioration in MTI capability. Other devices, such as surface wave and sonic delay lines, have been also employed, but in general, they suffer from the same general limitations.
Obviously, MTI cancellation can be conducted at video frequencies, however, another problem here presents itself, in that it is difficult to provide the required amount of delay in a device of reasonable size. Although video cancellers are inherently less critical in respect to the stability requirement, in that cycle-to-cycle cancellation is not involved at an RF or IF frequency, nevertheless, delay instability or drift deteriorates the quality of cancellation.
The optical sources, modulators, detectors and related components are well known of themselves. The aforementioned "Radar Handbook" text describes the state of the arts in respect to those devices, particularly in Chapter 37 thereof.
The manner in which the present invention deals with the problems in the prior art to provide a novel analog system with optical signal processing will be evident as this description proceeds.